DESCRIPTION OF THE PRIOR ART
Offshore platforms of the class described have been anchored to the ocean floor for a number of years, and more recently, it has been found necessary to position such platforms above greater depths of water. Normally, the platform structure is supported by steel pipe pilling driven through pile sleeves that constitute a part of the structure, and for reasonably shallow water depths, it is known to weld the pile to its surrounding sleeve at the working deck level. As water depths increase, the practice is to drive the pile through a sleeve positioned in the lower part of the structure and extending from the mud line, i.e. the sea bed, upward. The annular space between the outer surface of the pile and the inner surface of the sleeve is then filled with a cement grout which hardens to transfer the loading stresses from the sleeve to the pile. Thus, U.S. Pat. No. 3,857,247 discloses an offshore tower which is fastened to a sea bed by means of anchor piles driven down into the sea through tubular sleeves which are fixed to the bottom of the platform. After the anchor piles are driven, their upper ends, which are inside the sleeves, are locked to the sleeves by grout or similar material pumped into the clearance between the piles and the sleeves.
It will be appreciated that this technique is quite costly because, in addition to the substantial length of the sleeves which is necessitated in order to provide a large area for the grout interconnection, steel bars or rods, known as "shear connectors" are frequently welded to adjacent sleeve and pile surfaces to serve as keys for enhancing the locking action of the grout. Moreover, the use of grout for locking anchor piles to sleeves in depths of the order of 500 to 600 feet or more, has been difficult to accomplish in a reliable manner because the grout has to be pumped over a great distance and there is no reliable way of ascertaining whether the grout has fully filled the space between the sleeve and its associated anchor pile. As a result, the sleeve to pile connection is usually made considerably longer than would appear necessary in order to ensure an adequate margin of safety. The apparatus and piping needed to grout the annular space is also subject to mishap or failure, thus necessitating the provision of redundancy equipment. While the cost of these joints is considerable, even with the best of care satisfactory results are not always assured and the time required to pump the grout between the pile and the sleeve, and the time required for the grout to set, give rise to the possibility of disturbance of the grout during these periods, especially if heavy weather should shake the platform.
Various other efforts have been made to position offshore platforms by securing same to piles driven into the ocean floor as in U.S. Pat. No. 3,754,780 for example, which discloses a remote control, retractable locking clamp for detachably telescoping a cylindrical element of an offshore drilling tower with a subsurface structural cylindrical inner pile element. The clamp is fashioned in two parts with tapered overlapping ends positioned between the two cylindrical elements and encircling the pile element and its ends are hook shaped to receive therebetween a tapered pin movable by a reversible motor under remote control from the surface. The overlap is increased as the pin is moved further into the space between the hook shaped clamp end, thus causing the clamp elements to grip the outer sleeve elements detachably to lock the sleeve to the pile. Removing the tapered pin induces contraction of the clamp in order to unlock the sleeve from the pile.
A coupling apparatus useful for releasably securing various well components to one another is disclosed in U.S. Pat. No. 3,321,217 wherein clamping and unclamping is effected by shifting wedge rings under the control of reciprocal pistons, the wedge rings acting upon locking dogs movable radially of the elements to be connected in order to effect clamping.
In U.S. Pat. application Ser. No. 759,028 filed Jan. 13, 1977, there are disclosed arrangements whereby a structure is locked to an elongated anchor member. For example, an anchor member and the structure may be provided with mutually facing surface portions that converge toward each other in a downward direction. Wedges which are shaped to fit inside the space defined by converging surface portions are lowered into the space so as frictionally to engage these surface portions and a bias weight is then lowered onto the wedges to hold them in locking frictional engagement with the converging surfaces. The bias weight maintains the system in locked condition over extended periods of time.
Other teachings of interest are found in U.S. Pat. Nos. 2,989,326 and 3,847,493.